TLV320AIC31 Datasheet, PDF (27/70 Pages) Texas Instruments – LOW POWER STEREO AUDIO CODEC FOR PORTABLE AUDIO/TELEPHONY

TLV320AIC31
www.ti.com
SLAS497 – DECEMBER 2005
Table 2. Default Digital Effects Processing Filter Coefficients,
When in Independent Channel Processing Configuration
N0 = N3
27619
D1 = D4
32131
Coefficients
N1 = N4
–27034
D2 = D5
–31506
N2 = N5
26461
The digital processing also includes capability to implement 3-D processing algorithms by providing means to
process the mono mix of the stereo input, and then combine this with the individual channel signals for stereo
output playback. The architecture of this processing mode, and the programmable filters available for use in the
system, is shown in Figure 26. Note that the programmable attenuation block provides a method of adjusting the
level of 3-D effect introduced into the final stereo output. This combined with the fully programmable biquad filters
in the system enables the user to fully optimize the audio effects for a particular system and provide extensive
differentiation from other systems using the same device.
L
LB2
TOLEFTCHANNEL
LB1 Atten
TO RIGHT CHANNEL
R
RB2
Figure 26. Architecture of the Digital Audio Processing When 3-D Effects are Enabled
It is recommended that the digital effects filters should be disabled while the filter coefficients are being modified.
While new coefficients are being written to the device over the control port, it is possible that a filter using
partially updated coefficients may actually implement an unstable system and lead to oscillation or objectionable
audio output. By disabling the filters, changing the coefficients, and then re-enabling the filters, these types of
effects can be entirely avoided.
DIGITAL INTERPOLATION FILTER
The digital interpolation filter upsamples the output of the digital audio processing block by the required
oversampling ratio before data is provided to the digital delta-sigma modulator and analog reconstruction filter
stages. The filter provides a linear phase output with a group delay of 21/Fs. In addition, programmable digital
interpolation filtering is included to provide enhanced image filtering and reduce signal images caused by the
upsampling process that are below 20 kHz. For example, upsampling an 8-kHz signal produces signal images at
multiples of 8-kHz (i.e., 8 kHz, 16 kHz, 24 kHz, etc.). The images at 8 kHz and 16 kHz are below 20 kHz and still
audible to the listener; therefore, they must be filtered heavily to maintain a good quality output. The interpolation
filter is designed to maintain at least 65-dB rejection of images that land below 7.455 Fs. In order to utilize the
programmable interpolation capability, the Fsref should be programmed to a higher rate (restricted to be in the
range of 39 kHz to 53 kHz when the PLL is in use), and the actual Fs is set using the NDAC divider. For
example, if Fs = 8 kHz is required, then Fsref can be set to 48 kHz, and the DAC Fs set to Fsref/6. This ensures
that all images of the 8-kHz data are sufficiently attenuated well beyond a 20-kHz audible frequency range.
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